Electrostatic filter lens or mirror



Dec. 18, 1962 L. L. MARTON ETAL 3,069,592

ELECTROSTATIC FILTER LENS 0R MIRROR Filed March 10, 1960 John 14. 51%,050/1 BY ATTORNEYS 3,059,52 Patented Dec. 18, 1962 3,069,592 ELECTRGSTATIC FHLTER LENS (BR MHRRQR Ladislaus L. Mar-ton, Washington, D.C., and .iohn A. Simpson, Falls Church, Van, assigncrs to the United States of America as represented by the Secretary of Commerce Filed Mar. 10, 1960, Ser. No. 14,159 4 Claims. (Cl. 31517) This invention relates to an electron lens and particularly to an electrostatic filter lens or mirror wherein a comparatively low potential may be used on a remote element to control the fiow of an electron beam.

In various types of experiments involving electron scattering or appearance potential, it is desired to have an element whose electron transmission is critically sensitive to electron energy, and in various types of equipment such as a cathode-ray tube, it is desired to selectively control a beam of electrons.

. The arrangements in the prior art that provide an electron energy sensitive element by employing a conventional three-element electrostatic lens have several disadvantages. When a disc having an aperture smaller than 0.1 mm. is used, passing a sizeable electron beam through the aperture is difiicult and unreliable. When a larger aperture is employed, the energy resolution is lowered and a higher drive circuit is required. With either aperture an initially parallel beam comes out strongly divergent. Since the energy solution of the lens is dependent upon the size of the aperture, the smaller the aperture the greater the energy resolution. A smaller aperture, however, provides a shorter focal length resulting in greater divergence. Thus, in many applications the degree of resolution that can be obtained is dependent upon and limited by the focal properties of the lens. A typical lens of this type in the prior art has an energ resolution of 3 e.v. and a focal length of 2.6 mm.

When low drive is desired, in another prior art device a perforated disc, functioning as a grid, must be placed close to the cathode or other electron emitting structure which tends to alter the cathode loading.

Accordingly it is an object of the present invention to provide a filter lens wherein high energy resolution is obtained by accurately passing a beam of electrons through an aperture having a comparatively small diameter.

Another object is to provide a filter lens wherein the energy resolution is independent of the focal properties.

Another object is to provide an electron lens having an energy resolution below 0.1 e.v. and a focal length that can be varied from any desired value to infinity.

Another object of the present invention is to provide a filter lens wherein electrons passing through a potential barrier over a wide range of angles are returned to a collimated beam.

Another object is to provide an electrostatic lens wherein the fine collimation of an electron beam is not destroyed as transmission is varied from maximum to zero.

Another object of the present invention is to provide an electrostatic lens wherein a small potential diiference between an electron emitting structure and a remotely positioned element will control the electron beam intensity.

Other objects will become apparent from the following description of the annexed drawing, wherein:

The FIGURE is an embodiment of the present invention.

In accordance with the teachings of the instant invention, a pair of electrostatic immersion lens are placed back-to-back so that an input aperture is imaged in an aperture in a predetermined plane and re-imaged in the output. The energy of the electrons in the beam is decelerated'to a selected value on one side and reaccelerated on the other side of the plane to the initial energy. A small difference of potential between the predetermined plane and electron emitting structure will, therefore, control the flow of electrons; and since the size of the aperture in the plane controls the energy resolution of the lens and the proportion of the electrodes controls the focus, the degree of energy resolution is independent of focal properties.

Referring to the FIGURE, cathode 8 provides a beam 9 of essentially parallel electrons, shown here deflected by magnetic field 7, which is passed by aperture 10. The beam is subjected to an electric field established by electrodes 11, 12, and 14 which, circular in form, are shown in cross section in the FIGURE. A DC. power supply 13 is connected between cathode ii and disc 14 to control the potential difference between cathode and disc while the proportions, spacing, and positive potential applied to electrodes ll, 12 are selected, in accordance with well-known theories, to form an electrostatic immersion lens. Similarly, positive potential is applied to circular electrodes 17 and 18 which are proportioned and spaced to provide with the aid of disc 14 another electrostatic immersion lens. It is noted that there is a plane of symmetry through disc 14 and that the second immersion lens is the reverse of the first.

In practice, after the object and image positions of the lenses are selected for a desired application, the electrodes are proportioned according to existing theories and empirical knowledge of immersion lenses and the permissible values of such parameters as focal length and position of principal planes are determined from a set of empirical curves found, for example, in an article entitled, Systernatische Untersuehungen an Elektrostatischen Immersion Objektiven by Ernst-Adolf Soa in Ienaer Jahrbuch (1959), page 115.

After various combinations of the parameters are determined the one most desired from energy considerations is selected. For example, since the focal length should be as nearly independent as possible of the exact value of potential on disc 14, the combination of parameters is selected so that the change of focal length with change of potential on the disc is a minimum. If this were not done, a small variation of potential on the disc would change the size and position of the image in the plane of the disc relative to the width and position of beam 9.

In one embodiment wherein electrodes 1112, 17- 18 where constructed of nonmagnetic, stainless steel the following dimensions were found satisfactory for an energy resolution of less than 0.1 e.v. and a focal length of infinity. It is understood, of course, that the dimensions, potentials, and materials are given by way of example and not as limitations of the inventive concept.

Aperture 1t 0.040 Aperture 16 0.015 Diameter 29 0.750 Distances 21, 22 0.75 Distances 23, 24 0.180 Width 25, 26 0.25 Potential on electrodes 11 and 18 5 kv. Potential on electrodes 12 and 17 50 v.

All the dimensions are given in inches.

Although the embodiment is designed for 5 kv. operation, it could be scaled up or down as desired and the size of aperture 10 or 16 may be changed subject to known laws of electron optics. In some applications, for example, aperture 16 could be made larger than aperture 10 provided lower resolution is acceptable and a larger angle through which electrons pass through aperture 10 is de- 3 sired. It is understood, of course, that all elements, except DC. power supply 13, are located in a vacuum and that the embodiment disclosed will function equally well with charged particles other than electrons.

In operation, the embodiment disclosed in FIG. 1 may function as a lens and/or a mirror depending upon the setting of 13.0 power supply 13, i.e., the potential difierence between cathode 8 and disc 14. When the potential difierence is above a first level, all the electrons in beam 9 are passed by aperture 16; when below a second level, all are reflected by the potential barrier across aperture 16; and when the potential difierence has a selected value lying between the first and second level, electrons having energy greater than the selected value pass through the aperture while those having an energy level less than the selected value are repelled. In most instances these levels will be close to cathode potential, the first being slightly more positive and the second slightly more negative than the cathode. This latter mode of operation is shown in detail in the figure where the electrons in the image at the plane of aperture 16 having an energy level sufiiciently great to pass through the aperture are reacceierated to their initial energy and focused back to the parallel beam 27 by the electrostatic field shaped by disc 14, and electrodes 1718. The electrons in the image repelled by the potential barrier across aperture 16- are reaccelerated to a real image at infinity by electrodes 11- 12 and'disc 14. These electrons'are shown separated by magnetic field 7 into electron beam 28.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. While, for example, the arrangement disclosed has axially symmetrical geometry, for certain application two-dimensional geometry may be used, and although an initially parallel beam is accepted and emitted, any degree of divergence or convergence may be obtained by minor changes in electrode spacing and potentials. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In an electron energy sensitive element,- means for providing a stream of charged particles, means for decelcrating said charged particles in such a manner that a real image is provided in a selected plane, means for establishing a potential barrier in said plane having a magnitude such that the lowest energy particles which are transmitted through the plane have substantially zero kinetic energy, and means for accelerating the charged particles transmitted through said plane to a selected energy level.

2. In an electron sensitive element, means for providing a stream of charged particles including a terminal coupled to a region where the energy of the charged particles is substantially zero, means for decelerating said charged particles in such a manner that a real image exists in a selected plane, means positioned between said terminal and said plane for establishing a potential barrier in the plane having a magnitude such that the lowest energy particles which are transmitted through the plane have substantially zero kinetic energy, means for selectively controlling the magnitude of said potential barrier, and means for accelerating the charged particles transmitted through said plane to a selected energy level.

3. In an electron sensitive element having an input and an output, means for providing a stream of charged particles, a first aperture positioned in the input of said energy sensitive element for passing said charged particles, a second aperture located at the focal point of said first aperture, means for decelerating said charged particles in such a manner that a real image is provided in the second aperture, means for establishing a potential barrier across said second aperture having a magnitude such that the lowest energy particles transmitted through the second aperture have substantially zero kinetic energy, and means positioned between said second aperture and the output of said energy sensitive element for accelerating the charged particles transmitted through said second aperture to'a selected energy level.

4. In an electron energy sensitive element having an input and an output, means for providing a stream of charged particles including aterminal coupled to a'region where theenergy of'the charged particles is substantially zero, a first aperture positioned in the input of said energy sensitive element for passing said charged particles, a second aperture located at the focal point of-said'first aperture, means positioned between said first andsecond aperture for decelerating said charged particles in such a manner that a real image is provided in the second aperture, means for establishing a potential barrier across said second aperture having a magnitude such thatthe lowest energy particles transmitted through the second:

aperture have substantially zero kinetic energy, means for selectively controlling the magnitudeof said potential bar.- rier, and means positioned between said second aperture and the output of said energy sensitive element for accelerating the charged particles transmitted through said second'aperture to a selected energy value.

References Qited in the tile of this patent UNITED STATES PATENTS 1,810,018 Howes June 16, 1931 2,058,914 Rudenberg Oct. 27,1936

2,585,798 Law Feb. 12, 1952 2,718,610 Krawinkel Sept/20, 1955 FOREIGN PATENTS 733,345 Germany Mar. 25, 1943" 

